1. Field of the Invention
The present invention relates to an image pickup apparatus having an optical zooming function and an electronic zooming function.
2. Description of the Related Art
An image pickup apparatus, such as a video camera, is required for a high magnification zooming function for telephotographing. In order to realize the high magnification zooming that is unavailable only with the optical zooming, many image pickup apparatuses combine the optical zooming with electronic zooming that provides an electronic enlargement process of a part of a signal (image) from an image sensor.
For example, an image pickup apparatus disclosed in Japanese Patent No. 3,593,271 uses electronic zooming to increase an enlargement ratio at the wide-angle side, and selects optical zooming so as to increase the enlargement ratio after a predetermined enlargement ratio is obtained.
In addition, an image pickup apparatus generally provides an imaging aspect ratio of 16:9 instead of the conventional aspect ratio of 4:3. The image pickup apparatuses of this type often enable a user to select the imaging aspect ratio between 4:3 and 16:9.
In general, it is more effective for a good image quality to cut out more pixels or lines from all effective pixels of the image sensor, to reduce the cut images, and to output the reduced image to a monitor. However, in the telephotographing, the conventional image pickup apparatus preferentially uses the electronic zooming rather than the optical zooming to increase the enlargement ratio. The number of lines in the image output to the monitor when the optical zoom is being used is the same as that cut from the image sensor. In other words, in the telephotographing using the optical zooming, the line number cut from the image sensor satisfies the monitor's line number but the image quality of the output image degrades since the cutout line number is small. In addition, since the cutout line number from the image sensor changes when the imaging aspect ratio is switched, a relationship between the image quality and the imaging magnification changes due to the imaging aspect ratio.